The present invention relates generally to communication systems of coded data and, more specifically, to an improvement in determining an acquisition indicator bit at a receiver in a communication system.
Although the present invention will be described with respect to 3rd generation wideband code division multiple access (3G WCDMA) system, the same method can be used for determining an acquisition bit AIs at a receiver in other communication systems. General and specific references will also be made to the 3G WCDMA standard 3GPP TS 25.211, Physical channels and mapping of transport channels onto physical channels (FDD) (Release 4) and 3GPP TS 25.213, “Spreading and modulation (FDD)” (Release 4).
In 3rd generation CDMA systems, physical connections between UE (User Equipment) and base stations are established through the Physical Random Access Channel (PRACH) and Acquisition Indicator Channel (AICH). The UE transmits PRACH signal that carries the RACH preamble and message to a base station, to request a connection. When the base station recognizes a PRACH preamble, it responds with AICH to UE to indicate if the connection request is granted. An example of the AICH from 3GPP TS 25.211 is shown in FIG. 1.
Upon the AICH reception, the problem is to estimate the AIs based on the received real valued symbols a0, a1, . . . , a31 which are transmitted through AICH, where AIs, ∈{−1,0,1} and s=0,1, . . . , 15. An AIs of 0 indicates that the signature s is not a member of the set of available signatures. An AIs of 1 indicates a positive acknowledgement, and an AIs of −1 indicates a negative acknowledgement. At the base station, AIs is code multiplexed with others, {AIn:n≠s}, by the following formula:                                           a            j                    =                                    ∑                              s                =                0                            15                        ⁢                                                   ⁢                                          AI                s                            ⁢                              b                                  s                  ,                  j                                                                    ,                  j          =          0                ,        1        ,        …        ⁢                                   ,        31        ,                            (        1        )            where {bs,j:j=0,1, . . . 31} are given in Table 21 in 3GPP TS 25.211 and shown below in the example as BT.
The present invention is a method of determining an acquisition indicator bit AIs at a receiver in a communication system. The method includes receiving multiplexed acquisition indicator bits y=B×AI+n, where B is the signature matrix known at both base station and the receiver, AI is the set of all the acquisition indicator bits and n represents noise, for example, AWGN (additive white Gaussian noise). Next, an estimated value of the acquisition indicator bit AÎML(s) as a function of       ∑    j                   ⁢           ⁢                    B        T            ⁡              (                  s          ,          j                )              ×          y      ⁡              (        j        )            is calculated, where BT(s,.) is the s-th row vector of the transposed matrix BT for acquisition indicator bit AIs. Finally, the acquisition indicator bit AIs is set as follows:AIs=−1, if AÎML(s)<RAIs=0, if R≦AÎML(s)<UAIs=1, if AÎML(s)≧U,where R and U are decision thresholds.
The constants R and U may be equal absolute values, for example, −0.5 and 0.5, respectively. The estimated value AÎML(s) is calculated by       A    ⁢                   ⁢                            I          ^                ML            ⁡              (        s        )              =            1      32        ⁢                  ∑                  j          =          0                31            ⁢                           ⁢                                    B            T                    ⁡                      (                          s              ,              j                        )                          ×                              y            ⁡                          (              j              )                                .                    The method is performed without forming a matrix B or BT and is performed in software.
These and other aspects of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with accompanying drawings.